1. Field of the Invention
The present invention relates to a radiographic intensifying screen and, more particularly, to a radiographic intensifying screen comprising a support, phosphor layers provided thereon which comprise a binder and phosphor particles dispersed therein, and a protective film provided on the phosphor layers.
2. Description of the Prior Art
In a variety of radiography such as medical radiography for diagnosis and industrial radiography for nondistructive inspection, a radiographic intensifying screen is generally employed in close contact with one or both surfaces of a radiographic film such as an X-ray film for enhancing the radiographic speed of the system. The radiographic intensifying screen consists essentially of a support and a phosphor layer provided thereon. Further, a transparent film is generally provided on the free surface of the phosphor layer (a surface not facing the support) to keep the phosphor layer from chemical deterioration and physical shocks.
The phosphor layer comprises a binder and phosphor particles dispersed therein. When excited with a radiation such as X-rays transmitted through an object, the phosphor particles emit light of high luminance in proportion to the dose of the radiation. Accordingly, the radiographic film placed in close contact with the phosphor layer can be exposed sufficiently to form a radiation image of the object, even if the radiation is applied to the object at a ralatively small dose.
The radiographic intensifying screen having a protective film has adventages such that the phosphor layer is hardly scratched when the intensifying screen is in contact with a radiographic film or other devices and materials. For this reason, a radiographic intensifying screen employed in practice is generally provided with a protective film. However, the radiographic intensifying screen having a protective film has a drawback in that static electricity is likely produced between the protective film and a radiographic film, when the intensifying screen is set in close contact with the film. On the radiographic film sensitized by the radiographic intensifing screen having such static electricity, a static mark is apt to appear thereon. The static mark is produced in the form of an over-exposed portion on the radiographic film in contact with the intensifying screen, corresponding to the portion in which discharge of the static electricity takes place. The static mark appearing on the radiographic film is disadvantageous particularly in the medical radiography for diagnosis, because the static mark causes problems in the analysis of the resulting photographic image.
Terbium activated rare earth oxysulfide phosphors such as a terbium activated gadolinium oxysulfide phosphor (Gd.sub.2 O.sub.2 S:Tb) and a terbium activated gadolinium yttrium oxysulfide phosphor ((Gd,Y).sub.2 O.sub.2 S:Tb) emit light of high luminance when excited with a radiation such as X-rays, so that these phosphors have been heretofore employed as phosphors for a radiographic intensifying screen. The static mark is often observed on a radiographic film in the case of using radiographic intensifying screens employing these phosphors. The static mark is frequently observed on a radiographic film particularly when it is used in contact with the radiographic intensifying screen having a protective film made of polyethylene terephthalate (which film is widely employed as a protective film of the intensifying screen because it has various excellent characteristics such as high mechanical strength).
For preventing production of the static mark on a radiographic film, various measures such as coating an antistatic agent over the surface of the radiographic intensifying screen or incorporating an antistatic agent into the protective film have been used. However, high and lasting preventive effect on static mark is hardly obtained by these known measures. Accordingly, further improvement of the antistatic effect is desired particularly in the radiographic intensifying screen employing the above-mentioned terbium activated rare earth oxysulfide phosphor.
As described hereinbefore, a phosphor contained in the phosphor layer of the radiographic intensifying screen emits light (spontaneous emission) upon exposure to a radiation such as X-rays, and a radiographic film in close contact with the intensifying screen is exposed to the spontaneous emission. However, light continuously released from the phosphor after the exposure to the radiation is terminated, that is an afterglow, gives a photographic fog to an image formed on the radiographic film, extremely lowering the image quality (sharpness and granininess, etc.). Especially, the above-mentioned terbium activated rare earth oxysulfide phosphor shows relatively high afterglow characteristics, and when using a radiographic intensifying screen employing the phosphor, the photographic fog caused by the afterglow is apt to appear on the resulting image. Accordingly, it is also desired to improve the afterglow characteristics of the radiographic intensifying screen employing the above-mentioned phosphor.
Further, it is desired for a radiographic intensifying screen to exhibit a high radiographic speed and to provide an image of high quality. The same is true of the radiographic intensifying screen employing the above-mentioned terbium activated rare earth oxysulfide phosphor.